1. Field of the Invention
This invention is related in general to the field of liquid/solid separation. In particular, it relates to the use of hydrocyclones for separating bitumen froth from water and solids in the processing of tar sands.
2. Description of the Related Art
Hydrocyclones are used in nearly every water-based liquid/liquid and liquid/solid separation process, and in many particle classification circuits built in the world today, because of their high-volume capacity and simplicity of construction and operation. The feed stream is injected tangentially into the upper section of the hydrocyclone (called the head) to induce its rotation along the interior wall of the hydrocyclone and to produce centrifugal forces in the descending stream. These centrifugal forces decrease the time required to separate faster-settling, heavier liquids and/or coarser particles (as applicable), which migrate to the wall of the hydrocyclone, from slower-settling, lighter liquids and/or finer particles, which are transported to the center of the hydrocyclone by virtue of the drag forces created within the liquid. As a result, the lighter phases are collected overhead in the overflow effluent and the heavier phases at the apex of the hydrocyclone in the underflow effluent.
In liquid/liquid separation, such as in de-oiling processes for the recovery of liquid hydrocarbons from oily water streams, the performance of hydrocyclones is typically controlled by the so-called pressure differential ratio (PDR), defined as the ratio of the pressure differential in the overflow stream to that in the underflow. Referring schematically to the typical hydrocyclone arrangement of FIG. 1, PDR=(PF-O)/(PF-PU), where PF is the pressure of the feed stream F at the inlet of the hydrocyclone, PO is the pressure at the outlet of the overflow effluent O and PU is the pressure at the outlet of the underflow effluent U. By controlling the PDR of the process and maintaining it at the optimal level for the particular operation, the efficiency of separation is maximized and controlled. In essence, as operating conditions such as feed flow rate and pressure change over time, the PDR of the process is maintained at the level necessary to produce the desired liquid/liquid split between overflow and underflow of both light and heavy liquids. The control is achieved by adjusting the pressures of the overflow and underflow effluents to the levels necessary to maintain the target pressure differential ratio.
PDR has never been used in liquid/solid separation processes, probably because the underflow consists primarily of solids discharged at atmospheric pressure, typically in open vessels. Therefore, the idea of using pressure at the apex of the hydrocyclone as a control parameter is counterintuitive. However, the adaptation of PDR control to liquid/solid separation processes would be very desirable in the mineral processing industries, especially in the recovery of hydrocarbons from tar sands and in the fine classification of mineral particles.
The process of extracting hydrocarbons from tar sands produces a water slurry of bitumen and sand from which the bitumen is recovered. Typically, the mined tar sands are treated with large amounts of hot water (about 175° F.) to dislodge the bitumen from the sand granules, thereby producing a water slurry of sand that also contains a bitumen froth. Sometime the slurry is diluted with a light liquid hydrocarbon to dissolve the bitumen and reduce the overall viscosity of the hydrocarbon phase. The bitumen in the slurry needs to be separated from the undesirable water and solids for further processing into a synthetic crude oil and hydrocyclones have been used successfully in such separation process. However, because the density difference between the hydrocarbon and water phases is much smaller than that between the water and solids, the efficiency of hydrocarbon recovery with conventional hydrocyclones is less than optimal.
Therefore, the present invention is directed at applying the principle of PDR control to liquid/solid separation processes, in particular as applicable to the recovery of bitumen from tar sands and similar processes, in order to improve the efficiency of the process.